Automotive vehicles often have an automatic transmission located in a powertrain that delivers power from an engine to traction wheels of the vehicle. When the vehicle accelerates from a standing start, the transmission automatically changes the relative ratio of a transmission input shaft that receives power from the engine and a transmission output shaft that delivers power from the transmission to downstream elements of the powertrain and eventually to the wheels. The ratio changes are generally performed by selectively braking components of interlinked planetary gear sets or selectively engaging components of the gear sets to other components of the gear sets by the use of friction elements. The gear sets are mounted in a housing that also contains actuators for the friction elements. A pump is used to supply automatic transmission fluid to the friction element actuators to enable them to perform the gear changing function and also provides fluid to the gear sets so that they are properly lubricated.
It is considered desirable in the art to ensure that automobile transmissions allow the complete powertrain to be as efficient as possible. However, because of adjustments required to properly manage transmission operation when the temperature of the automatic transmission fluid is elevated, efficiency can be compromised. The elevated temperature leads to lower viscosity in the automatic transmission fluid which, in turn, leads to reduced pump efficiency and to more fluid leakage as the fluid progresses around the transmission. In order to maintain a required lubrication flow and fluid pressure to supply the friction element actuators so that they may control the friction elements in a timely and effective manner, a minimum allowable pump speed must be increased. Since the pump usually obtains power from the engine, a minimum allowable engine speed must also be increased. Increasing the minimum allowable pump and engine speeds increases fuel consumption and worsens fuel economy.
Generally, prior art solutions to the problem of supplying sufficient automatic transmission fluid that has an elevated temperature were directed to requiring a minimum engine speed that would provide an adequate supply of fluid under worst-case temperature conditions. These solutions had the advantage of simplicity in that once the minimum allowable engine speed was set no further control was necessary. Also, the minimum engine speed required by other factors, such as drivability, vehicle noise and harshness, was frequently higher than the minimum required due to increased automotive fluid temperature so the increased temperature was not a major factor when trying to reduce engine speed. However, in order to reduce parasitic loss of power caused by the transmission fluid pump, transmission designers are reducing transmission pump displacements which, in turn, is requiring higher pump speeds. Thus, the need to provide an adequate supply of transmission fluid can become a controlling factor on minimum engine speed.
Other prior art solutions have been directed to increasing fuel efficiency by addressing different problems. For example, Japanese Patent Document JP 4066337 discloses an oil pump that is directly connected to an engine. The idle speed of the engine is increased when the oil temperature reaches a certain level. Basically, this arrangement is not concerned with setting a minimum engine speed during transmission gear ratio shifts, but rather focuses on adjusting engine idle speed.
Another prior art solution is set forth in U.S. Pat. No. 5,556,349 which discusses a known automatic transmission having a normal temperature shift pattern and a high temperature shift pattern. The goal is to have the automatic controller constantly monitor the transmission fluid temperature and prevent it from overheating by switching to the high temperature shift pattern. The high temperature shift pattern avoids heating the transmission fluid as much as the normal temperature shift pattern in that the high temperature pattern shifts to a higher gear at a higher speed than the normal temperature pattern. This increases torque converter average speed and reduces torque converter average torque, both of which changes reduce the amount of heat generated, particularly by an open torque converter.
As can be seen by the above discussion, there is a need in the art for a system that will effectively reduce the minimum allowable engine speed requirement during gear shifts, while still providing adequate amounts of automatic transmission fluid needed for lubrication and for friction element actuators.